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https://hdl.handle.net/2144/37822

Abstract

Background: Mirror imaging in identical twins has long been noted and suggests that left-right asymmetry may become established early in embryogenesis. However, it is not known whether the clinical reports of “mirroring” in twins are annulled by an equal number of cases lacking mirroring. As left-right patterning is a key component of laterality-based birth defects, it is important to determine whether aspects of left-right asymmetry are in fact set prior to the splitting event that produces monozygotic twins. We aimed to determine whether significant mirror imaging occurs in transverse facial asymmetries in monozygotic and dizygotic twins.
Material and Methods: The sample included PA cephalograns from 56 pairs of monozygotic twins and 57 pairs of dizygotic twins from the Forysth/Moorrees Twin Study (females age 14-1 5 and males age 15-16). The films were digitized and anatomical landmarks identified. Using Geometric Morphometric analyses including Procrustes superimposition, the landmark configuration of one individual twin was reflected (mirrored) and superimposed using Procrustes superimposition. Principal components analysis (PCA) and MANOVA tests 1V were performed to determine the differences between monozygotic and dizygotic twins. A secondary Procrustes superimposition was then conducted without reflection. If mirroring asymmetry was present, the average Procrustes distance within reflected twin sets (D1) would be smaller than those superimposed without reflection (D2). T tests were performed to determine the differences between reflected and non-reflected regions in monozygotic and dizygotic twins.
Results: After reflection, no statistically significant differences were found for any regions (with the exception of Mandible A Right, p=0.0258) between monozygotic and dizygotic twins. When comparing reflected versus non-reflected regions, Midface D Left and Mandible C Right in dizygotic twins yielded negative values for D1-D2; however permutation tests revealed these values are not significant. T tests showed 15 out of 20 regions had significant smaller mean values for D2 versus Dl in monozygotic twins, while only 7 out of 20 in dizygotic. A Z test comparing these two proportions revealed this difference between twin types is significant (p=0.011), With the monozygotic twins having significantly more regions that fit better when non-reflected than the dizygotic twins.
Conclusion: No statistically significant differences in mirroring were found between monozygotic and dizygotic twins in any craniofacial regions except for Mandible A Right, in which dizygotic twins showed a better fit when mirrored. Upon examination of the differences between reflected and non-reflected regions, monozygotic twins showed a statistically significantly greater number of regions that fit better when superimposed versus reflected in comparison to dizygotic twins. From this study we conclude that no significant mirroring occurs in craniofacial asymmetries, perhaps due to the biological stability of neural crest cells that derive the cranial cartilage and skeleton.